1. Field of the Invention
The present invention relates to a lithium cobaltate based positive electrode-active material for a lithium secondary cell having excellent energy density and a method of manufacturing same.
2. Discussion of the Background
Along with the recent rapid progress in electronic equipment toward portable and cordless domestic appliances, the use of lithium secondary cells as power sources for such compact electronic appliances has also increased. In 1980, Mizushima et al. reported the usefulness of lithium cobaltate as a positive electrode-active material for a lithium secondary cell "Material Research Bulletin" vol. 15, p. 783-789 (1980)!, and since then, research and development efforts regarding lithium cobaltate based positive electrode-active materials have been made, resulting in many proposals on the subject to date.
These conventional proposals cover techniques for achieving a high energy density for a positive electrode-active material such as, for example, one using the chemical composition LixCoO.sub.2, where, 1.05.ltoreq..times..ltoreq.1.3, of lithium cobaltate, thereby achieving a material rich in lithium (Japanese Patent Laid-Open No. 3-127454), one achieving in contrast a higher cobalt concentration by using the chemical composition LixCoO.sub.2, where, 0.ltoreq..times..ltoreq.1, (Japanese Patent Laid-Open No. 3-134969), ones doping metal ions such as Mn, W, Ni, La and Zr (Japanese Patent Laid-Open Nos. 3-201368, 4-328277, 4-319259 and 4-319260), and one setting out residual Li.sub.2 CO.sub.3 of up to 10 wt % in lithium cobaltate (Japanese Patent Laid-Open No. 4-56064).
On the other hand, the known techniques requiring specific physical features of the lithium cobaltate based positive electrode-active material include one using an LiCoO.sub.2 specific area of up to 2 m.sup.2 /g (Japanese Patent Laid-Open No. 4-56064), one using amorphous LiCoO.sub.2 (Japanese Patent Laid-Open No. 5-21066), ones imparting specific particle size properties to LiCoO.sub.2 (Japanese Patent Laid-Open Nos. 4-33260 and 5-94822), and ones setting forth LiCoO.sub.2 crystal grains having a specific X-ray diffraction intensity (Japanese Patent Laid-Open Nos. 3-272564 and 5-36414). Diverse and various proposals for methods of manufacturing a lithium cobaltate based positive electrode-active material include, for example, Japanese Patent Laid-Open Nos. 3-285262, 4-249074, 4-123762, 5-54886, 5-54888, 5-62678 and 5-182667.
In general, lithium cobaltate used as a positive electrode-active material for a lithium secondary cell is manufactured, for example, by mixing a lithium salt such as lithium carbonate and a cobalt compound such as cobalt oxide so as to give an atomic ratio of Li/Co within a range of from 0.9 to 1.2, and firing the resultant mixture at a temperature within a range of from 600.degree. to 1,100.degree. C. Physical and chemical properties of the resulting LiCoO.sub.2 delicately and diversely vary with such firing conditions as the heating rate and the firing atmosphere, which exert considerable effects on performance of the positive electrode-active material including such characteristics as discharge capacity and charge and discharge cycle.
The present inventors carried out extensive studies, placing emphasis on the foregoing phenomenon, and confirmed that a composition with a slight content of free cobalt oxide in lithium cobaltate could display excellent energy density performance, and that such a composition could be manufactured under specific firing conditions.
The present invention was developed on the basis of the foregoing findings, and is aimed at providing a lithium cobaltate based positive electrode-active material for a lithium secondary cell having high energy density and a method of industrially manufacturing same.